In recent years, metal belt type (push-belt type and chain type) or toroidal type continuously variable transmissions have been developed and practically used as transmissions for automobiles, etc. As a lubricating oil for such continuously variable transmissions, a lubricating oil for automatic transmissions was initially used. However, as continuously variable transmissions have been improved in their performance, lubricating oils for them have been also required to show an improved performance. In particular, lubricating oils for automatic transmissions are mainly required to provide optimized frictional characteristics of wet clutches. Therefore, when such lubricating oils for automatic transmissions are used for continuously variable transmissions, a metal to metal frictional coefficient is liable to be insufficient so that a problem is caused in that it is difficult to provide a large capacity of torque transmission.
In order to solve the afore-mentioned problem, lubricating oil compositions for continuously variable transmissions having a high metal to metal friction coefficient are developed. For example, Patent Document 1 discloses a lubricating oil composition in which a sulfur-based extreme pressure agent, a phosphorus-based extreme pressure agent and an alkaline earth metal-based detergent are compounded, Patent Document 2 discloses a lubricating oil composition which has a specific range of a friction coefficient and in which a metal salt-based detergent and a zinc dialkyldithiophosphate are compounded, and Patent Document 3 discloses a lubricating oil composition in which one or more of polymethacrylates, alkaline earth metal phenates and alkaline earth sulfonates are compounded together with an imide compound and a zinc alkyldithiophosphate. These lubricating oil compositions have a high metal to metal friction coefficient but have a problem in that, for example, wear resistance is reduced by corrosion.
Patent Document 4, on the other hand, discloses a lubricating oil composition in which at least one of phosphoric acid esters and phosphorous acid esters, at least one of thiophosphoric acid esters and thiophosphorous acid esters, and a polyamine compound are compounded. This lubricating oil composition is considered to have good wear resistance. However, the metal to metal friction coefficient thereof is desired to be further improved.
As described in the foregoing, the conventional lubricating oil compositions are not fully satisfactory with respect to achievement of both high metal to metal friction coefficient and excellent wear resistance and, therefore, are desired to be further improved in their performance.
[Patent Document 1] Japanese Patent Application Publication No. H9-100487
[Patent Document 2] Japanese Patent Application Publication No. H11-80772
[Patent Document 3] Japanese Patent Application Publication No. H11-181464
[Patent Document 4] Japanese Patent Application Publication No. 2002-155292